So I was re:watching re:zero today and then that talk between Subaru/good-Satella happened.
As she listed everything that Subaru did for her, I realized that it is only possible for him to do all that if he never died and we all know they won't spend a single day together without him dying at Elsa's hands.
My theory is that during their first meeting ever Emilia#0 did the same thing as present Emilia and introduced herself to Subaru as Satella, then they went on merrily adventuring together but with a twist: No one wanted to kill them.
We already know that all the trials Subaru had to go through up to that point were orchestrated by Roswall under the guidance of Echidona, so I thought to myself: What if that's exactly what happened the first time around, they adventured together and all that good stuff... Then, at some point and for reasons I can't figure out Echidona decided to screw them over, she altered the past, causing Subaru to die and Emilia decided to take the Envy witch factor so that she could save him effectively becoming the first Satella, screwing up Echidona's plans and that's the real reason why she hates Emilia so much.
How come Satella exists in the past then? That's the power of her authority. She exists in the past, present and future.
As to how Echidona altered the past, I'm assuming she can write Roswall's gospel from the future.
What do you think? Help me improve this theory and fill the gaps